1. Field of the Invention
This invention relates to prevention of fouling of surfaces in aquatic environments by microscopic and macroscopic organisms. More particularly, antifouling agents, compositions, coatings and methods for repelling and preventing attachment, growth and proliferation of biofouling organisms are disclosed.
2. Description of Related Art
Biofouling organisms settle on surfaces that are submerged in aquatic environments. Submerged surfaces such as water pipes, power plant water intake systems, sewer pipes, boat hulls, heat exchangers, grids, and the like are prone to biofouling. Biofouling is a major problem for most industries involved with fresh or salt water environments. Aquatic pests frequently clog pipes or become attached to submerged surfaces thus interfering with normal operations. For example, warm water associated with power plant cooling systems provides an ideal environment for the attachment and growth of aquatic organisms. Biofouling organisms also attach to other surfaces which contact aqueous solutions such as fishing nets, buoys, pilings, off-shore platforms, lumber, roofs, and concrete.
When a clean surface is introduced into an aquatic environment, it typically becomes coated with a conditioning layer of hydrophobic dissolved organic compounds. Microorganisms such as bacteria, algae, fungi, and protozoa attach to the conditioning layer and establish colonies which result in the formation of a slime layer. Such slimes can cause problems, e.g., by significantly reducing heat transfer across exchangers in cooling systems. Furthermore, slime layers contribute to the establishment of biofouling communities because planktonic (free floating) larvae of many invertebrate biofouling organisms are physically and chemically attracted to the slime layer. Examples of invertebrate biofouling organisms include mollusks such as mussels and oysters, and crustaceans such as barnacles. The release of specific compounds from the slime layer can also trigger metamorphosis of the planktonic larvae (see Hadfield (1986) Bull. Mas. Sci. 39:418-425 and Young and Mitchell, (1973) Int. Biodeterior. Bull. 9:105-109).
The blue mussel, Mytilus edulis, presents a particular problem at coastal power plants located in the Northeastern region of the United States. Mytilus edulis planktonic larvae settle on and attach to any available substratum. More recently, zebra mussels have begun to clog structures submerged in fresh water or brackish water environments. Settled juveniles grow rapidly and form dense aggregates which cause such problems as clogging inflow or outflow pipes.
Biofouling of underwater structures such as power plant water intake systems and heat exchangers results in significant economic losses to industry. Decreased fuel efficiency, increased cleaning and maintenance expenses, as well as outage expenses all contribute to increased economic expenditures. The incentive for preventing marine biofouling is great. As a result, various methods and compositions have been developed for prevention of marine biofouling. For example, utilities employ several methods for removing established biofouling communities. Periodic power outages are employed to physically enter power plant systems to remove organisms and debris. In addition, utilities often attempt to kill established biofouling communities by pumping large volumes of chlorine and molluscicides through water handling systems. However, these methods are slow acting and adversely affect the local ecology downstream from the effluent. Furthermore, these chemical treatments are inefficient because toxins are mixed in bulk water phase in an attempt to treat a surface phenomenon. Certain organisms such as the blue mussel can sense sub-lethal concentrations of some toxins and seal themselves off for long periods thereby effectively preventing contact with the toxins. Therefore, another drawback of certain existing chemical treatments is that relatively large toxic doses must be maintained for extended periods to effectively eliminate biofouling pests.
Ablative toxic antifouling coatings containing tributyl tin, copper alloys, mercury compounds, or cathodic protection have also been employed to control fouling. These antifouling coatings may include toxins which are leached into the aquatic environment to inhibit biofouling. The following examples of antifouling coatings are included for purposes of illustration. U.S. Pat. No. 5,096,488 describes a vinyl polymer or copolymer emulsion containing certain enumerated ammonium compounds. U.S. Pat. No. 5,116,407 describes an antifouling marine coating containing certain enumerated amine compounds acting as paint binders and marine biocides. U.S. Pat. No. 5,143,545 describes an antifouling marine paint containing certain enumerated water insoluble antibiotics said to be toxic to gram negative organisms of the genus Oceanospirillum, and a metallic compound, i.e., copper, tin, or zinc, acting as a marine biocide. U.S. Pat. No. 5,199,977 describes an antifouling paint containing a polymeric metal containing hybrid salt and certain enumerated organic ligands.
Observations have been made that certain sea creatures are associated with bioactive compounds. Attempts have been made to determine whether specific sponges are associated with compounds that have antimicrobial activity. Thompson et al., Marine Biology 88, 11-21 (1985), describe screening and bioassays for biologically active substances from sponge species near California, USA. Various extracts and metabolites are described as being biologically active but none of the substances was active in all assays.
A preemptive antifouling composition is needed for treating surfaces in aquatic environments which is highly effective and (1) does not contain heavy metals or synthetic toxins that adversely affect the local ecology, (2) is easy to manufacture and incorporate into or on undersea structures, (3) is easily cleaned and (4) has a prolonged effective lifetime. The benefits associated with such a composition would be enormous.